1. Field of the Invention
This invention relates to water dilutable coating compositions useful as a "leveler" (i.e. "surfacer" or filler") coating in multilayered coatings, especially for use in the automotive industry, based on aqueous polyurethane dispersions. It also relates to a process for the production of the leveler compositons and to their use in the automotive inudstry.
2. Statement of Related Art
At the begininng of the age of the motor car, painting was a time consuming, complex process in which several different paint layers were successively applied with repeated interruptions through long drying times and rubbing processes [see the chapter "Automobile Finishing" in Nylen, et al., Modern Surface Coatings, Interscience (pub.) New York, 1965. ]
In the nineteen twenties, this procedure was superceded by quick drying nitrocellulose (NC) paints. NC paints in the automotive industry were replaced after the Second World War by amine-neutralized alkyd resins which were in turn partly replaced or complemented by acrylate resins.
A modern paint line for painting car bodies comprises succesive stations for: degreasing, phosphating, cathodic electrodeposition (CED), undersealing, leveler coating application, and surface coating. In the case of metallic paints, the surface coat in turn comprises a base layer and a finishing clear varnish [see Goldschmidt, et al., "Lackieren von Metall und organischen Materialien" in Glasurit-Handbuch, 11th Edition, Hannover 1984. ]
In modern automobile painting, the leveler or filler performs a particular function. It smoothes out any unevenness and roughness from the CED on the metallic substrate and prepares the substrate for the following surface coat. Since it is desireable to reduce the need for labor intensive rubbing as far as possible, particularly good levelling of the surface is required.
In addition, the leveler as the second surface coat of the paint finish, is intended to complement and enhance the corrosion inhibiting properties of CED. To perform this function, stone throw resistance, resistance to other mechanical influences and adhesion to the substrate have to satisfy particularly stringent requirements.
In addition to adhesion to the substrate, the leveler also should impart good adhesion to the surface coat or to the base layer of a metallic paint. Furthermore, in modern high speed paint lines, the leveler is required to dry rapidly and thoroughly, even in thick layers, at the usual stoving temperatures of 120.degree. to 170.degree. C.
In the majority of autmobile factories, the levelers used are solvent containing stoving lacquers. For economic reasons and to reduce environmental pollution, efforts are being made to avoid organic solvents in coating compositions as far as possible.
U.S. Pat Nos. 4,489,135 and 4,558,090, and corresponding European patent document No. 89,497 describe coating compositions containing a polyurethane dispersion, for forming the base layer of a metallic paint. However, there is a considerable technical difference in terms of application and requirement profile between the base layer of a metallic paint and a leveller coating.
U.S. Pat Nos. 4,489,135 and 4,558,090 describe polyurethane dispersions which contain a linear polyether and/or polyester diol having a molecular weight of from 400 to 3000.
The basic principle of preparing water dilutable polyurethane dispersions by incorporation of a 2,2-di(hydroxymethyl)-carboxylic acid in a polyurethane chain containing at least one other polyol is described in U.S. Pat. No. 3,412,054. The various possible methods for producing polyurethane ionomers are comprehensively presented in Dietrich et al., in Angew. Chemie 82, 53 (1970). However, this disclosure is not sufficient for formulating polyurethane dispersions which are generally compatible with water dilutable polyesters and which are also suitable for specific use as leveler coatings.
The "oil-modified" polyurethane dispersions described in U.S. Pat. No. 4,277,380 teach away from the present invention, because "oil-modified polyurethane dispersions" are understood to be products which, besides diisocyanate, contain only dimethylolpropionic acid or other polyol components and unsaturated "fatty acid ester polyols" (defined as polyesters of unsaturated triglycerides or unsaturated fatty acids and low molecular weight polyfunctional alcohols) as the polyurethane components.
U.S. Pat. No. 4,555,564 (and corresponding German patent document No. 33 18 595) describe aqueous alkyd resins compositons for surface coating.